1. Field Of The Invention
The present invention is concerned with catalysts useful for the treatment of gases to reduce contaminants contained therein. More specifically, the present invention is concerned with improved catalysts of the type generally referred to as "three-way conversion" or "TWC" catalysts. TWC catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing both oxidation and reduction reactions, such as the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. Such catalysts find utility in a number of fields, including the treatment of the exhaust gases from internal combustion engines, such as gasoline-fueled automobile and other spark-ignition engines.
2. Background and Related Art
In order to meet governmental emissions standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants in vehicle and other engine exhaust gases, so-called catalytic converters containing suitable catalysts are emplaced in the exhaust gas line of internal combustion engines to promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") and the reduction of nitrogen oxides ("NO.sub.x ") in the exhaust gas. Two separate catalyst members or beds can be used in series, for example, the first to promote reduction of NO.sub.x and the second to promote oxidation of HC and CO, with optional oxygen (air) introduction between the beds. Alternatively, a single bed TWC catalyst, which substantially simultaneously promotes both oxidation and reduction as described above, may be used, provided that the air-to-fuel weight ratio ("A/F ratio") of the engine whose exhaust is being treated is held close to the stoichiometric ratio. For the foregoing purpose, catalysts comprising one or more platinum group metals and, optionally, base metal oxides distended upon a high surface area, refractory oxide support are well known in the art. The support may comprise a high surface area alumina coating carried on any suitable carrier such as a refractory ceramic or metal honeycomb structure, as well known in the art. For example, see C. D. Keith et al U.S. Pat. No. 4,552,732. Such high surface area alumina materials are generally referred to in the art as "gamma alumina" (although it is usually a mixture of the gamma and delta phases and may also contain eta, kappa and theta phases) or "activated alumina". Such alumina, when fresh, typically exhibits a BET surface area in excess of 60 square meters per gram ("m.sup.2 /g"), often up to about 200 m.sup.2 /g or more. It is a known expedient to stabilize such activated alumina supports against thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or, most usually, rare earth metal oxides, for example, ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al U.S. Pat. No. 4,171,288.
It is known in the art to provide two catalyst members in series to treat noxious pollutants in an exhaust gas. For example, U.S. Pat. No. 3,896,616 of C. D. Keith et al discloses an arrangement for treating exhaust gases from internal combustion engines in which an initial catalyst of relatively small volume is placed as close to the exhaust manifold of the engine as possible with a second, larger volume catalyst positioned further along the exhaust pipe beneath the vehicle. The small volume of the initial catalyst and the higher temperature of the exhaust gases closer to the engine cause the initial catalyst to heat up very quickly, thereby commencing purification of the exhaust gas during the start-up period when the downstream catalyst, because of its lower operating temperature, is still relatively ineffective. The Patent discloses that the art has contemplated by-passing the initial catalyst after the subsequent or downstream catalyst is heated to operating temperature (see column 3, lines 4-19). However, the Patent teaches continuing operation of the initial catalyst after the engine start-up period in order to reduce the amount of nitrogen oxides in the engine exhuast gas. In order to accomplish this result, an additional, extraneous fuel is introduced between the initial and subsequent catalysts. (Column 4, line 34 et seq.) The initial catalyst may also be supplied, during the engine start-up period, with either a fuel or an oxygen-containing gas, depending on operating conditions. (See column 4, lines 3-24.)
As disclosed at column 9, line 22-46 of the Patent, the initial catalyst may comprise an activated alumina support in which one or more platinum group metals, preferably including a catalytically effective amount of palladium, are disposed. The subsequent catalyst is disclosed as of similar composition, comprising one or more platinum group metals, especially platinum or palladium, and other ingredients such as base metals including iron group metals. (See column 12, lines 21-35.)
German Offenlegungsschrift 36 08 292 A1, A. Konig et al discloses a catalytic converter for treating internal combustion engine exhaust comprising a first converter (5) containing a multifunction catalyst and a second converter (6) with a nitrogen oxide reducing catalyst, and including the introduction of air via line 7 between the catalyst stages. The Patent discloses a system in which the second converter is a NO.sub.x reduction catalyst, for example a zeolite or coke support containing a base metal oxide, such as titania or vanadia, disposed thereon. The up-stream catalyst is a conventional three-way conversion catalyst and, in accordance with the invention, NO.sub.x and ammonia emanating from the first catalyst are converted in the second catalyst. The publication discusses that in retrofitting automobiles which do not contain oxygen sensor probes and other equipment necessary to control the air-to-fuel ratio within the narrow range necessary for good conversion rates using a three-way conversion catalyst, the disclosed arrangement provides a means for converting the nitrogen oxides as well as ammonia which may emanate from the first catalyst, especially when the air-to-fuel ratio is not closely controlled. The second catalyst also advantageously contains an oxygen storage component, such as a zeolite, so that ammonia produced in the first converter during fuel-rich operation can be stored in the second converter and, other subsequent fuel-lean operation, be used for the reduction of nitrogen oxides.